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   难度：Medium
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   <h1 class="question_title">
    105. Construct Binary Tree from Preorder and Inorder Traversal
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   <p>
    Given preorder and inorder traversal of a tree, construct the binary tree.
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    <strong>
     Note:
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    <br>
    You may assume that duplicates do not exist in the tree.
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    For example, given
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preorder =&nbsp;[3,9,20,15,7]
inorder = [9,3,15,20,7]</pre>
   <p>
    Return the following binary tree:
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   <pre>
    3
   / \
  9  20
    /  \
   15   7</pre>
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    105. 从前序与中序遍历序列构造二叉树
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   <p>
    根据一棵树的前序遍历与中序遍历构造二叉树。
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    <strong>
     注意:
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    <br>
    你可以假设树中没有重复的元素。
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    例如，给出
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   <pre>前序遍历 preorder =&nbsp;[3,9,20,15,7]
中序遍历 inorder = [9,3,15,20,7]</pre>
   <p>
    返回如下的二叉树：
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   <pre>    3
   / \
  9  20
    /  \
   15   7</pre>
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